Electronic distance measuring technology is incorporated into a mechanical caliper for measuring thicknesses of soft biological tissues, thereby eliminating friction and inaccuracy inherent in a purely mechanical caliper and achieving adjustable and precisely repeatable contact forces where the caliper touches the tissues.
A standard test for immune sensitivity is to inject a tiny amount of test material into the ear of a mouse. If, after a time, the ear becomes edematous, the test is considered positive. A similar test is done on rat foot-pads. The normal way to measure this edema has been to use an Oditest gauge, a European-made precision caliper with a dial face graduated in 100ths of a millimeter as shown in FIG. 1. This device was made for measuring hard materials such as metal, plastic, paper, etc., and works very well for such purposes. However, it applies a significant force, squeezing whatever it measures, which is not ideal for soft biological tissues because the squeezing action reduces the thickness and diminishes the resulting measurement.
A larger problem is that the squeezing force applied to the test tissue by the Oditest gauge is not consistent from measurement to measurement. An ideal gauge should apply the same force for every measurement at a given thickness. However, when the present inventors set up an Oditest gauge on a test stand, the Oditest did not do that. The force seemed to depend partly on how fast the operator released the actuator lever on the gauge. A faster release gave more force. But no matter how carefully the actuator lever was released, the Oditest gauge applied forces varying by as much as 30% from measurement to measurement.
Inside an Oditest gauge is a mechanical arrangement of pivots, gears, a spring, and a string wound around a spool. It is believed that friction between these parts is responsible for the force variations.
The present invention eliminates most of the internal mechanical parts of the Oditest gauge and substitutes therefor an electronic distance measuring device, thereby avoiding most of the friction believed to cause the problems described above in connection with the prior art device. Tests conducted by the present inventors show that, for a given measured distance, the device of the present invention applies precisely the same force every time the measurement is taken, by reason of use of a spring to provide the sole force pressing the caliper jaws against the biological tissue.
Accordingly, the present invention provides an electronic caliper for measuring the thickness of an object, e.g., the paw or ear of a laboratory animal, which electronic caliper includes a housing, a fixed caliper arm fixed to the housing and extending from the housing to a fixed free end external to the housing and providing a first jaw, and a movable caliper arm pivotally mounted on a pin fixed to the housing and extending from the pin to a movable free end also external to the housing and providing a second jaw facing the first jaw. A spring, mounted on or in the housing serves to bias the first and second jaws together with the object to be measured held therebetween. A non-contact electronic sensor is mounted in or on the housing to face the movable caliper arm for measuring the position of a point thereon with the jaws closed on the object to be measured. The electronic caliper further includes opening means for opening the jaws against the force of the spring.
In another aspect the present invention provides a method for measuring the extent of swelling in an ear, paw or other member of a laboratory test animal utilizing the electronic caliper described above. The method involves opening the jaws of the electronic caliper and placing the ear or paw between the open jaws. The jaws are then allowed to close on the ear or paw with the force pressing on that member limited to that provided by the spring alone. The degree of swelling of the ear is determined based on the measured position of the point on the movable caliper arm facing the non-contact electronic sensor.